1. Field of the Invention
The present invention relates to a zoom lens system, an imaging optical device, and a digital appliance. More particularly, the present invention relates to a compact zoom lens system suitable for use in a digital appliance (for example, a interchangeable-lens digital camera) equipped with an image input capability for capturing an image of a subject with an image sensor, an imaging optical device that outputs in the form of an electrical signal the image of the subject captured with the zoom lens system and the image sensor, and a digital appliance incorporating such an imaging optical device so as to be equipped with an image input capability.
2. Description of Related Art
Patent Document 1 listed below proposes a zoom lens system that is compact enough to be suitable for use in a mirrorless exchangeable-lens camera or a video camera and that offers an angle of view of about 100 degrees (2ω) combined with a zoom ratio of about 2×. Proposed in Patent Document 1, however, is a two-group zoom lens system, which does not provide sufficient flexibility to secure flatness of the off-axial image plane over the entire zoom range and which thus leaves comparatively large curvature of field uncorrected.
By increasing the number of movable lens groups for zooming, it is possible to increase flexibility and thereby solve the above-mentioned problem in performance. For example, Patent Document 2 discloses a zoom type composed of four, namely negative, positive, negative, and positive, lens groups, wherein the distances between those lens groups vary during zooming. This four-group zoom type, compared with the two-group zoom type of Patent Document 1, provides more flexibility in securing image plane flatness over the entire zoom range.
Patent Document 1: Japanese Patent Application Publication No. 2010-250233
Patent Document 2: Japanese Patent Application Publication No. 2011-257776
The zoom lens system disclosed in Patent Document 2, however, is designed for single-lens reflect cameras, which require a long back focal length; accordingly, to obtain a high degree of retrofocus, a strong convergent power needs to be given, with the result that the fourth lens group, which contain a comparatively large number of lens elements, needs to be moved out greatly toward the object side for zooming to the telephoto end. Thus, this zoom lens system cannot be said to be compact enough for use where no swing-up mirror is needed as in a mirrorless interchangeable-lens camera or a video camera.